The present invention relates generally to a semiconductor device, and more particularly, to a capacitor which inhibits leakage current, and a method for forming the same.
As an integration degree of semiconductor devices is increased and a design rule is sharply reduced, a capacitor capable of more capacitance within a limited area is required. A Dynamic Random Access Memory (DRAM) device, in which a cell transistor and a cell capacitor form a unit memory cell, requires larger capacitance for improved memory operation. To increase capacitance, a storage node is formed in a cylindrical shape to increase an effective surface area of the capacitor. In addition, a material having a higher dielectric constant k is introduced into a dielectric layer of the capacitor.
A height of a cylindrical storage node is increased to increase capacitance of the capacitor, but a process margin in a photolithography and exposure process or an etch process narrows with the increase of the height. Increasing the cylinder height thus is limited by this insufficiency of the process margin. Also, decreasing an effective thickness of the dielectric layer of the capacitor has been attempted to increase the capacitance, but reliability of the capacitance may be lowered since leakage current may be increased with the decrease of the thickness of the dielectric layer.
To decrease the effective thickness of the dielectric layer of the capacitor or increase the effective dielectric constant of the dielectric layer, the dielectric layer of the capacitor is formed of a single dielectric material. However, when depositing a layer of a single dielectric material, it is difficult for the deposited layer to have a uniform thickness due to a geometric structure of the storage node. For example, for forming the storage node of a cylindrical shape, the deposition is superiorly carried out on an end of an upper portion of the cylindrical node when depositing the dielectric layer. In contrast, relatively little of the dielectric material is deposited at the bottom corner of the cylindrical node. Accordingly, the thickness of whatever dielectric material is deposited on the bottom corner of the cylindrical node is relatively thin and, thus, the thickness uniformity of the dielectric layer is reduced as a whole. Because the thickness of the dielectric layer is relatively thin on the bottom corner of the cylindrical node or the side wall and bottom adjoining the corner, an electric field is relatively concentrated to these portions. Such partial concentration of the electric field may cause a leakage current phenomenon in which charges present in the node leak to the weak portions.
To inhibit the leakage current due to the thickness non-uniformity in the dielectric layer, the dielectric layer can be a structure in which several dielectric material layers are stacked. For example, an aluminum oxide (Al2O3) layer having an amorphous structure between dielectric layers and thus known to be capable of reducing the leakage current can be a leakage current preventing layer. However, when forming the dielectric layer with a multilayer, the thickness of the dielectric layer becomes relatively thick compared to a dielectric layer having a single layer. Also, as various kinds of dielectric material layers are stacked, the effective dielectric constant of the entire dielectric layer is reduced compared to the dielectric constant k of the dielectric material. Therefore, the capacitance of the capacitor is decreased. As a result, development of a method capable of inhibiting generation of the leakage current and a thinner effective thickness of the dielectric layer is required for increasing the capacitance.
Also, use of a conductive material as a cell plate having a large difference in a work function from the dielectric layer may inhibit the leakage current between the dielectric layer and the cell plate. However, when this cell plate is in direct interfacial contact with the dielectric layer, oxygen may diffuse from the oxide forming the dielectric layer and thus leakage current properties may be deteriorated. Also, when an interfacial adhesive force between the dielectric layer and the cell plate is weak, the cell plate may be lifted during succeeding processes.